The present invention relates to sound reproducing devices for producing stereophonic sound in narrow spaces, such as the passenger compartment of an automobile. More particularly, the invention relates to an improved sound reproducing device of this general type in which the stereo effect is enhanced.
When the listener listens to sound from such a sound reproducing device in the passenger compartment of an automobile, he feels as if the sound is coming from a narrow room. This is the so-called "closed-room effect". Furthermore, in the passenger compartment of an automobile, the listener is near the the loudspeakers so that he hears sound coming directly from the loudspeakers. That is, he hears the sound as if he were near the sound source. This increases the closed-room effect even more.
Therefore, heretofore, in order to eliminate the above-described problems, signals having frequency components in middle and high sound frequency ranges, which give a high directional effect, were applied mutually to the opposite channels to decrease the directional effect and to thereby increase the acoustic spreading effect. In each channel, a signal obtained by delaying frequency components in a high range are applied to provide a reverberation effect, thereby to reduce the closed-room effect.
A conventional sound reproducing device which utilizes the above-described reverberation and spreading effects will be described with reference to FIG. 1.
In FIG. 1, reference character 1L designates an input terminal to which is applied a signal L (left) in a two-channel stereo system, 1R an input terminal to which is applied a signal R (right), 2 an addition circuit in which a signal -R which is obtained by inverting the phase of the signal R is added to the signal L, 3 a filter circuit, 4 a voltage amplifying or attenuating circuit for changing the amplitude of the signal L-R which has passed through the filter circuit 3, 5 an addition circuit in which the output of the circuit 4 (i.e. a signal L.sub.1 -R.sub.1) is added to the signal L from the input terminal 1L to output a signal L+L.sub.1 -R.sub.1, 6 an addition circuit in which a signal obtained by inverting the phase of the output signal L.sub.1 -R.sub.1 from the circuit 4 is added to the signal R from the input terminal 1R to output a signal R+R.sub.1 -L.sub.1, 7 a high-pass filter circuit, 8 a phase shift or delay circuit, 9 an addition circuit in which the output signal L+L.sub.1 -R.sub.1 from the addition circuit 5 is added to the output signal L.sub.2 -R.sub.2 from the phase shift or delay circuit 8 to provide a signal L+L.sub.1.sup.- +L.sub.2 -R.sub.1 -R.sub.2, and 10 an addition circuit in which a signal obtained by inverting the phase of the output signal L.sub.2 -R.sub.2 from the circuit 8 is added to the output signal R+R.sub.1 -L.sub.1 from the addition circuit 6 to output a signal R+R.sub.1 +R.sub.2 -L.sub.1 -L.sub.2.
Further in FIG. 1, reference character 11L designates a power amplifier circuit for the signal L+L.sub.1 +L.sub.2 -R.sub.1 -R.sub.2, 11R a power amplifier circuit for the signal R+R.sub.1 +R.sub.2 -L.sub.1 -L.sub.2, and 12L and 12R left and right loudspeakers, respectively. In FIG. 1, the signs (+) and (-) beside the addition circuits 2, 5, 6, 9 and 10 are intended to mean direct addition of a signal without phase inversion, and addition of a signal after its phase has been inverted, respectively.
As is apparent from FIG. 1, the two channel signals in the circuit are symmetrical. That is, when either of the two signals passes to the channel opposite to its own, its phase is inverted, but when it returns to its own channel, the phase is restored.
The operation of the above-described circuitry for providing the reverberation and spreading effects will be described.
The phase of the signal R applied to the input terminal 1R is inverted and the inverted signal is added to the signal L from the input terminal 1L in the addition circuit 2, as a result of which a difference signal component L-R, representing the difference between the right and left channel signals, is provided. The amplitude of the signal L-R, after the signal passes through the filter circuit 3, is set to a suitable value by the voltage amplifying or attenuating circuit 4. The difference signal component L-R includes a relatively large amount of reverberation component. This reverberating component is extracted by the addition circuit 2. From the reverberation component, the difference signal L.sub.1 -R.sub.1 having a frequency component in a range of about 100 Hz to 1.2 KHz, to which the ear is especially sensitive, is filtered by the filter circuit 3 where a frequency component which produces a stronger reverberation effect is emphasized by a resonance circuit in the filter circuit 3. The signal L.sub.1 -R.sub.1 is added to the signal L in the addition circuit 5. Furthermore, the signal L.sub.1 -R.sub.1 is added to the signal R in the addition circuit 6 after its phase has been inverted. As a result, a reverberation effect and, especially, an acoustic spreading effect are provided by the sound from the loudspeakers 12L and 12R.
Next, the signal L.sub.1 -R.sub.1 having a relatively large reverberation component, after passing through the circuit 4, is applied to the high-pass filter circuit 7, which is adapted to damp a low frequency component, where a signal component in a relatively high frequency range and having a strong directional effect is extracted. The signal thus extracted is applied to the phase shift or delay circuit 8 where it is subjected to phase inversion or time delay to provide a signal L.sub.2 -R.sub.2. The signal L.sub.2 -R.sub.2 is added to the signal L+L.sub.1 -R.sub.1 in the addition circuit 9 and added to the signal R+R.sub.1 -L.sub.1 in the addition circuit 10 after its phase has been inverted. Because of the phase inversion and the time delay, an acoustic spreading effect and, especially, a reverberation effect are produced by the sound from the loudspeakers 12L and 12R.
As is apparent from the above description, the directional effect is suppressed to enhance the acoustic spreading effect of sound emanated from the loudspeakers. Phase inversion or time delay is used to increase the reverberation effect to thereby decrease the closed-room effect.
However, it should be noted that, in the conventional sound reproducing device, the reverberation effect and the acoustic spreading effect are fixedly set and accordingly cannot be varied by the listener to suit his individual preferences.